We Still Don’t Know What Causes That Knuckle Cracking Sound

Snap, crackle, pop: the sound of cracking knuckles is familiar to everyone, but scientists are having a hell of a time figuring out what causes it. A new ultrasound imaging study offers additional clues, but no definitive answers.

The GIF above was created from ultrasound footage collected in a first-of-its-kind knuckle cracking study led by radiology professor Robert Boutin of the University of California, Davis. Boutin’s team imaged the hands of 40 subjects as they pulled their knuckles a total of 400 times (62 of those pulls resulted in cracks). Turns out, the sound of a person cracking his or her knuckles is often accompanied by a bright flash at the knuckle base, a region doctors refer to as the metacarpophalangeal joint (try saying that one five times fast).

At a meeting of the Radiological Society of North America in Chicago this week, Boutin and his team argued that these metacarpal “fireworks” are related to the formation of gas bubbles—but whether and how those bubbles are connected to the crackles we hear is not yet clear.

Believe it or not, scientists have been trying to figure out what happens when joints crack since the 1940s. Two leading explanations for the pops we hear: one, that they’re caused by the formation of gas bubbles due to decreased pressure around the joints, and two, that cracking your knuckles actually causes tiny gas bubbles to burst. Over the years, scientific studies have accumulated evidence in support of both theories—bubble forming and bubble bursting—and to this day, the debate continues to rage.

The latest imaging results seem to lend evidence to the bubble forming theory. “We’re confident that the cracking sound and bright flash on ultrasound are related to the dynamic changes in pressure associated with a gas bubble [forming] in the joint,” Boutin said in a statement. However: based on the timing of the flash and cracking sound, it isn’t clear whether one causes the other. More work is needed to assess the relationship between bubble formation and audible knuckle pops, Boutin says.

In the meanwhile, next time you crack your knuckles, take an extra moment to appreciate the tiny fireworks you’ve just set off, and the fact that many brilliant minds are attempting to figure out what in the heck is causing that popping noise.